Survey terminology The CHE should be familiar with four terms that are used in the context of epidemiological surveys 1. Screening refers to examining the population 2. Prevalence looks at how widespread a condition is in the population. 3. Incidence how often a condition occurs. 4. Distribution assesses where most disease occurs in the population INDICES An index (plural indices) is a numerical method of measuring data in a survey or in individuals. Before indices were devised, oral hygiene and the extent of disease were graded by terms, such as good, average or poor. Obviously, this was unsatisfac- tory as one person's idea of good may differ from another. Factors such as the extent of plaque and calculus were not recorded.